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Variable Stars

Given a large enough time frame, all stars vary in brightness. However, some stars vary in shorter time frames, such as days, months or even years. Hence the name variable star. These variations can be due to many different factors. Astronomers have divided variable stars into two main groups: Extrinsic and Intrinsic variables which are separated from each other depending on what causes the variation. In the case of extrinsic variables, the change is because of an eclipse or rotation of the star. It follows that extrinsic variables have two subgroups: Eclipsing Binaries, and Rotating Variables. In the case of the latter, the star has some extremely large sunspots on the surface, which are usually less bright than the rest of the surface.
Intrinsic variables are stars which alter their brightness due to some physical conditions in the stars themselves. Intrinsic variables have three subgroups: Catalysmic, Eruptive and Pulsating variables.
Cataclysmic variables are binary stars, in which one component is a white dwarf. It may receive so much mass from the donor star that it explodes as a supernova. However, it may also receive mass which will suddenly ignite a fusion reaction on the surface of the white dwarf. In this case the explosion will be a less dramatic, but still very violent one: a nova! A nova explosion does not destroy the star, so multiple novae are possible from the same system.
Eruptive variables have giant flares on their surface which make the star brighten, or they may loose mass in the form of ejections. Wolf-Rayets (massive OB-stars), are famous for this kind of behaviour.
Pulsating variables are usually red giants that expand or contract their radius, due to internal fluctuations of pressure. In the process the star varies in brightness.

There are many subgroups of variable stars, but one famous type is a Cepheid variable, which is a pulsating type. Their brightness varies very regularly and astronomers use them to measure distances across the universe!

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